The present invention is directed to a device for maintaining alignment between two rotating coordinate systems. While it has broader utility, it will be described herein in the context of satellite communications, and more particularly in the context of an improved technique for the tracking of a geosynchronous satellite by a ground antenna.
Most communication satellites are intended to be "geostationary", i.e., they orbit the earth at the same angular velocity as the spin of the earth so that they remain substantially stationary with respect to any given point on the earth. This allows the satellites to be used on a 24 hour per day basis.
However, if the orbital path of a satellite is even slightly inclined with respect to the equator, the satellite will appear from a vantage point on earth to move in a figure eight or oriented perpendicular to the geosynchronous arc with a sinusoidal period of 24 hours. An earth station antenna communicating with the satellite can be oriented at the center of the figure eight path, but will nonetheless experience signal strength variations over the course of a sidereal day.
There is a need, then, for an improved tracking technique which will accommodate the figure eight path of a geosynchronous satellite to obtain improved signal strength for a given earth station antenna size, and a further need for such a technique which can be implemented with low cost and high reliability.
On a more generalized level, it will be appreciated that there is a need for a technique for maintaining alignment between two coordinate systems which are rotating at the same rate but at some arbitrary angle with respect to one another.